...what is the chemical reason for this.
I'm not gonna hug you an sh*T...and I do think you could be a little quicker on the search function, and less on the posting for many of your questions...but that's neither here nor there...
The problem is you are trying to think of this in terms of Chemistry...Or at least in-organic chemistry (or like mixing koolaid; ) 2 dormant chemicals producing a reaction (like vinegar and baking soda.) That can pretty much be controlled, and calculated...
1 Ounce of Bicarbonate of Soda added to 1 fluid ounce of Vinegar produces X ounces of co2...
Although it SEEMS like it with beer, because we add x amount of sugar to the bottling bucket...it isn't really the case.
In brewing, carbonation has more in common with how you and I, and everyone on here eat and "process waste," than mixing vinegar and Bicarb. But as opposed to how we produce "certain compounds" (trying to be PC here) INCLUDING methane (hint, hint) the yeast produce Alcohol and CO2.
Different diets , different digestive systems, and different amounts of food will produce different "results," as well as the environmental conditions of where the organism is living in (Warm=more active, more hungry, more "waste" produced Cold=less active, less hungry, less "waste" produced.)
The kind of wast we want is
CO2 absorbed back into the beer for carbonation. That means AFTER THE CO2 IS PRODUCED...it needs to be reabsorbed into the beer...the video I linked to on the blog shouws, what I call false carbonation.. That many new brewers swear that their 3 day old beer is carbed....It isn't...The beer is gassy, but the gas hasn't maxed out the headspace then gone back into the beer.
The kind of food, is the type of sugar used, as well as the type of fermentables already present after fermentation, the different digestive systems are the different types of yeast we use (they all have their own "eating habits." )
Also the size of the vessel can play a role as well....That's why it takes longer for a pint or a bomber to carb and condition than a 12 ouncer. (BK once explained it really well somewhere iirc.)
That's why if you play with numbers in beersmith, like, base style of beer, temp of beer at bottling time, temp of beer at storage, amount of priming sugar, size of bottle, type of priming sugar used (corn, brown, dme, krauzen,) you will get different volumes of co2 produced.
But even then it is a crapshoot...we are dealing with living things with their own timeframe and agendas...
That's why for "da n00bs" we recommend the "rule of thumb" 4-5 ounces of cprn sugar, 3 weeks @ 70 degrees...We know that it works in the ball park...maybe with an Efficiency of 80-90% of the time....good enough, and we can trouble shoot the rest.
But did you know that it is theoretically (at least from some books, and some number crunching in beersmith) to bottle carb your beer without any priming sugar at all? Just bottle, stick it in the closet and come back in a few months to drink...I can't say that I want to wait that long, nor would I be sure that the amount of carb produced would be enough to condition the beer enough to taste good.
(Remember "fizzyness" does a lot for the taste of something, as well as the body. That's way BMC's are so higly carbed....let one go flat and give it a taste...You'll see what I mean.)
Take a look at this post, from a highly contentious thread where a guy wanted to not prime some beer...I had ran some numbers on an esb, you can see how the different numbers affect the "output."
https://www.homebrewtalk.com/797980-post33.html
Hope this helps, clear it up for you....